The present invention relates to the tantalum capacitor art and more particularly to the production of tantalum wire for use as leads to tantalum powder capacitors. In the production of tantalum capacitors, tantalum powder is compressed to a pellet, the pellet including a tantalum lead wire, the resultant green pellet with the associated lead wire is then subjected to a sintering operation, normally under a vacuum, to create a metallurgical and electrical bond between the individual powder grains and to the lead wire. Thereafter, the resultant sintered body is anodized and impregnated with an electrolyte, preferably solid, and encapsulated to form the finished capacitor. It is essential that there be a good electrical and metallurgical bond between the tantalum lead wire and the capacitor pellet. It is also essential that the tantalum lead have sufficient mechanical strength and flexibility to withstand the rigors of further fabrication and attachment (which is often done on automatic machinery) to other circuit elements.
As circuit miniaturization has advanced, the need for smaller capacitors has also developed. Capacitor miniaturization has progressed to the point where many are of a diameter smaller than 2.5 mm and the capacitor leads are as small as 0.25 mm in diameter. This has been due to the improvement in obtaining higher capacitance per unit weight of tantalum powder. As capacitors become smaller, the percentage of value in the lead wire becomes larger so that with the smallest capacitors the lead wire is almost 50% of the capacitor value.
Another problem with capacitor miniaturization is that the necessary small diameter of the capacitor lead wire is such that at the elevated temperatures employed in the sintering operation, grain growth in the tantalum lead wire can be sufficiently great for the grain size to equal the tantalum wire diameter. This makes a very brittle wire. In the past this problem of grain growth has been addressed by various means to inhibit the grain growth in the wire. Such inhibitors are oxides, nitrides, and various alloying constituents. This has a disadvantage that these inhibitors are difficult to control and expensive to add and may also interfere with the metallurgical and electrical characteristics of the tantalum lead wire as well as perhaps contamination of the tantalum powder itself. Contamination introduced by the use of finer powder sizes which contain higher values of oxygen would also tend to further embrittle the lead wire.